One downside of the 578 is that it's microphone has a built-in CPU to handle all of the functions on the microphone, i.e. this is a proprietary microphone design.  There is NO PTT line to use.  But this guy was a someone laborious workaround.  If you know of another way please jump in!

http://tim-yvonne.com/ham/dmr/how-to/signalink-to-d578.htm

73 Steve